The present invention relates to an exposure technology and, more particularly, to a technology which is effective if applied to a technology for fabricating a phase shift mask to be used in an exposure step or one step of a process for fabricating a semiconductor integrated circuit device and a semiconductor integrated circuit pattern transfer technology using the phase shift mask fabricating technology.
In the photolithography step of transferring a circuit pattern on a photo mask by using a light of g-th line or i-th line, there arises a limit to the minimum working size of a pattern to be excellently transferred, as the degree of integration of the elements of a semiconductor integrated circuit device increases.
As a method of reducing the minimum working size of the pattern, it is conceivable to shorten the wavelength of the light used for the exposure. But, various problems practically arise in the shortening of the wavelength of the light. As a method of improving a resolution with the wavelength being held as it is, there is another technology for increasing the numerical aperture (NA) of an optical system in an exposure apparatus.
In accordance with the use of a light of short wavelength and the improvement in the NA, however, there arises a problem that the focal depth becomes extremely shallow. In recent years, therefore, various exposure technologies for improving the resolution while retaining the focal depth have been examined and are represented by a phase shifting method.
The phase shifting method is a technology for preventing the contrast of a projected image from degrading by changing the phase of the light to transmit through a photo mask.
The phase shifting technology is disclosed in Japanese Patent Laid-Open No. 173744/1983, for example. Here, there is disclosed a phase shifting technology (of Levenson type), in which a transparent film is disposed on one of a pair of transparent areas interposing a shielding area, to establish a phase difference between the lights having transmitted through the two transparent areas at the time of exposure so that the interfering lights may be weakened in the portion which is the intrinsic shielding area on a semiconductor wafer.
In Japanese Patent Laid-Open No. 67514/1987, for example, on the other hand, there is disclosed a phase shifting technology (of auxiliary shifter type). According to this technology, after a fine aperture pattern has been formed in a portion of a shielding area on a photo mask, a transparent film is disposed either in the aperture pattern or in a transparent area close to the aperture pattern, to establish a phase difference between the light having transmitted through the aperture pattern and the light having transmitted through the transparent area close to the aperture pattern, thereby to prevent the amplitude distribution of the light having transmitted through the transparent area from being transversely spread.